1. Field of the Invention
The present invention relates to the manipulation of plasma characteristics in a particle beam source. More specifically, the present invention provides the capability to produce a generally homogenous, quiescent plasma having a preselected, adjustable plasma potential VPA.
2. Description of the Related Art
Devices using beams of particles created from a plasma source have achieved wide utility in many well-known applications, including electronic devices and semiconductor manufacturing processes. However, the inherent instability and nonuniformity of materials in the plasma state have always plagued the performance of typical plasma sources. Even a so-called xe2x80x9cquiescentxe2x80x9d plasma generally has local nonhomogenous areas throughout its volume, as ions are constantly produced and lost through recombination. The major, inner, portion of a quiescent plasma is substantially space-charge neutralized with the net mutual repulsion between like-charged species balanced by mutual attraction between oppositely charged species. This means, for any charged particle that is well-separated from the boundary of the plasma but having a trajectory toward the boundary of the plasma, a force will be exerted on the plasma which tends to pull it back toward the plasma. Therefore, most of the inner volume of the plasma can be regarded as generally homogeneous.
However, within this population of charged species the electrons are far more mobile than the ions. Therefore, the electrons tend to leave the ions at the boundary of the plasma, creating a slightly greater population of ions near the plasma boundary. In addition, repulsion forces between ions at the plasma boundary tends to accelerate some of the ions outwardly, with such acceleration decreasing with increasing distance from the boundary of the plasma. Simultaneously, as electrons get farther from the ion-rich plasma boundary, their acceleration increases. These conditions are effectively reversed when the boundary of the plasma is near a conductive surface, which tends to return electrons to the plasma and to accelerate ions causing the surface to be negative relative to the plasma and the plasma adjacent to the surface to be positive. This voltage differential is called the plasma potential.
The capability of a plasma to produce accelerated ions has been useful in many applications, including semiconductor manufacturing applications such as Plasma-Enhanced Chemical Vapor Deposition (PECVD), anisotropic Plasma Dry Etching, cleaning, and removal of polymer resist (ashing). In these devices, ions are directed against the surface of a semiconductor structure (e.g. a wafer which may or may not have layers or other structures formed thereon) for purposes of implanting, depositing or etching a material. In addition, the Neutralizer Grid Patent describes etching and cleaning methodologies using a high-energy neutral particle beam created from accelerated ions that pass through a grid and become neutralized by shallow angle elastic surface forward scattering. In either approach, an accelerated ion beam must be extracted from a plasma source by heating the plasma and/or artificially increasing its potential, and then deflected and focused upon the workpiece. However, it is typically more difficult to manipulate an ion beam than an electron beam, since the increased mass of ions (relative to electrons) requires much higher levels of energy. At the same time, precise control of the beam characteristics in an ion beam device is even more important than it is in an electron beam device, since the crystal structure of the semiconductor material is much more easily damaged by the collision of relatively massive ions or neutral particles, even at relatively low velocities, as compared to electrons. Indeed, it is usually necessary to anneal a semiconductor material after an ion implantation operation to restore the crystal lattice structure and repair damage thereto caused by the kinetic energy of the particles used in the implantation process.
Another problem that has plagued typical ion-beam source devices relates to the ability to maintain a coherent ion beam. As described above, it is desirable to keep the overall energy of the accelerated ion beam as low as is necessary to achieve the desired result, to minimize the inevitable damage to the semiconductor""s crystal structure that the ion beam will cause. When the ion beam energy is lowxe2x80x94on the order of 50 to a few hundred eVxe2x80x94the ion beam must be space-charge neutralized to keep the beam sufficiently coherent to avoid a drastic drop in beam intensity as the beam propagates to the workpiece, and to avoid an undesirable charging effect on the workpiece. This means that a sufficient number of electrons must be introduced into the ion beam, such that the overall charge of the beam in a certain volume of space is neutral. In the absence of these electrons, the repulsion forces between the ions in the beam will cause the beam to quickly diverge and lose intensity.
One method that those in the art have used to introduce electrons into an accelerated ion beam to neutralize the space-charge is to insert an electron source into or near the beam, such as a stand-alone hot filament that emits thermionic electrons. U.S. Pat. No. 4,361,762 to Douglas and the patents referenced therein describe various neutralization techniques and their associated problems that primarily relate to the complexity of the apparatus required and the difficulty of controlling the electron emission rate to achieve space-charge neutralization. Douglas discloses a method and apparatus that uses a closed-loop feedback circuit to control a filament array for space-charge neutralizing an ion beam. While Douglas"" apparatus addresses the control difficulty issue, the apparatus still adds undesirable complexity to the plasma source generator to achieve the required beam neutralization
The present invention solves the plasma stability problems described above by providing a stable and uniform quiescent plasma that is effectively separated from the primary plasma region. The present invention can produce a high-quality, homogenous quiescent plasma having a user-selected, adjustable artificial plasma potential from any primary plasma, thus obviating the need for a high-quality primary plasma in these types of applications. In addition, the present invention solves the ion beam coherency and neutralization problem because it produces a space-charged neutralized plasma beam that effectively comprises an equal number of accelerated ions and electrons per unit of volume, without the need for additional equipment or control electronics.
The present invention comprises an RF-powered plasma accelerator/homogenizer that produces a quiescent plasma having a generally homogenous preselected plasma potential VPA from a primary plasma. The plasma accelerator/homogenizer includes an RF-conductive accelerator/homogenizer structure that includes a plurality of dielectric-coated accelerator/homogenizer surfaces having a total surface area ARF. The RF-conductive accelerator/homogenizer structure is reactively coupled to an RF source using a coupling device. The RF source produces an RF voltage within the accelerator/homogenizer structure that causes thermal electrons from the primary plasma to be absorbed by the dielectric coated accelerator/homogenizer surfaces that are quasi-uniformly dispersed throughout the primary plasma. The present invention also includes a containment assembly that holds the quiescent plasma at the generally homogenous preselected plasma potential VPA. The containment assembly includes an RF-grounded structure having a total ground surface area AG, where ARF greater than AG. The RF-grounded structure is separated from the accelerator/homogenizer structure by a dielectric material. The coupling device may comprise one or more variable vacuum capacitors, or an RF tuning circuit that incorporates stray capacitance associated with a plasma liquid cooling system coupled to a pick-up electrode adjacent to a dielectric spacer in an arrangement that has a preselected characteristic capacitance, or an impedance-controlled circuit that couples to the RF-conductive accelerator/homogenizer structure using the stray capacitance of the primary plasma, or an RF matching network. The RF voltage produced inside the accelerator/homogenizer structure oscillates around a positive offset voltage determined by (ARF/AG)x, where x comprises a positive number not greater than 4. The preselected plasma potential VPA is approximately equal to the value of the offset RF voltage when the value of the offset RF voltage is positive.
In addition, the present invention is an accelerated ion beam generator that produces an accelerated ion beam by from a quiescent plasma created by diffusing a heated primary plasma through an accelerator/homogenizer structure. The accelerator/homogenizer structure has a uniform voltage potential VB and a total surface area ARF. The RF-conductive, dielectric coated surfaces of the accelerator/homogenizer structure are quasi-uniformly dispersed throughout the primary plasma, oriented in a direction generally parallel to the direction of travel of ballistic electrons from the heated primary plasma. VB can be developed by tapping RF power from the power source that heats the primary plasma, by a separate RF power source reactively or directly coupled to the accelerator/homogenizer structure, or by an external DC voltage source.
The quiescent plasma develops a generally homogenous preselected plasma potential VPA that is approximately equal to VB. An RF-grounded structure having a total ground surface area AG, wherein ARF greater than AG, attracts ions from the quiescent plasma to produce the accelerated ion beam.